1) Field of the Invention
The present invention relates to a digital writing optical system, and more particularly, to an optical writing unit of a solid optical writing system, an image forming apparatus, a process cartridge, and a method of adjusting light intensity.
2) Description of the Related Art
Due to the recent downsizing of digital image outputting apparatuses like a digital copying machine, a printer, and a digital facsimile, there is a demand of achieving a compact optical writing unit that performs digital writing. Digital writing systems are roughly classified into two kinds at present. One is the optical scan system that causes a light deflector to optically scan with a flux of rays emitted from a light source like a semiconductor laser and forms a light spot using a scanning/image forming lens, and another one is the solid writing system that causes an optical element array to form a light spot from a flux of rays emitted from a light-emitting-element array like a light emitting diode (LED) array or an organic electro-luminescence (EL) array.
The optical scan system has a long optical path due to the scanning with light by the light deflector, whereas the solid writing system has an advantage of making it possible to construct the optical writing unit compact, because the optical path can be made very short.
The solid writing system has another advantage such that because of having no movable parts such as a light deflector, noises can be suppressed.
Japanese Patent Application Laid-Open Publication No. H2-62257 discloses a technique of LED drive timing control to make the amounts of light emission of an LED uniform.
Japanese Patent Application Laid-Open Publication No. H4-305667 discloses a technique of achieving a uniform spot width of the light intensity at a predetermined threshold.
Japanese Patent Application Laid-Open Publication No. H1-227254 discloses:
(1) A characteristic point in the emission intensity distribution of a light emitting element is measured, and light-intensity correction data for supplying energy to the light emitting element is determined based on the characteristic feature;
(2) Temporary light-intensity correction data is determined based on the fluctuation of the light intensity and is corrected based on the characteristic feature to thereby determine light-intensity correction data;
(3) The characteristic point is a change in peak position;
(4) The characteristic point is a change in peak value; and
(5) The characteristic point is a change in emission light size.
Japanese Patent Application Laid-Open Publication No. 2002-127492 discloses that the amounts of light emission of light emitting elements are set such that the result of comparison of the property values of the light emitting elements at the predetermined threshold in the exposure intensity distribution lies within a preset range over the entire effective image area.
However, the optical writing unit of the solid writing system that comprises a light-emitting-element array having multiple light emitting elements, and an optical element array causes fluctuation of the light spot on an image carrier (e.g., a photoconductor) due to fluctuation of the amounts of light emission of the light emitting elements or fluctuation of the shape of the optical element array. In this case, the “fluctuation” means “fluctuation of intensity”, “positional fluctuation”, “fluctuation of the spot shape” or the like. Therefore, an image output from an image forming apparatus that uses the optical writing unit as an exposure unit has density irregularity and cannot acquire excellent images.
To acquire excellent images by the conventional optical writing unit, there is a proposal such that density irregularity is reduced by performing correction, such as constant light intensity correction to make the amounts of exposure to a photoconductor constant or constant spot size correction to make the size of a light spot to be formed on a photoconductor constant at a predetermined threshold.
The constant light intensity correction sets the amount of the supply current so as to ensure a constant amount of exposure by measuring the amounts of exposure to a photoconductor using a light intensity measuring unit and changing the flow rate of the current to be supplied to each light emitting element. In general, the amount of the current to be supplied is controlled based on light-intensity correction data of 4 to 6 bits, and the amount of light emission is set within a range of several %, though the light intensity is constant.
The constant spot size correction sets the amount of the supply current so as to ensure a constant light spot size by measuring the size of a light spot to be formed on a photoconductor using a spot size measuring unit and changing the flow rate of the current to be supplied to each light emitting element. Since the amount of the current to be supplied is controlled based on light-intensity correction data of 4 to 6 bits, as per the constant light intensity correction, there is a limit to the control amount even with the constant spot size.
The Japanese Patent Application Laid-Open Publication No. H11-227254 further proposes the following. With the constant light intensity correction performed using temporary light-intensity correction data, a light spot size Wi for each light emitting element is measured. It is then determined whether the light spot size Wi depicts a convex change upward, and when there is a convex change upward, temporary light-intensity correction data of the i-th light emitting element is corrected. This system still has the following problems:
(1) Temporary light-intensity correction data is corrected only at an area that fulfills the determination requirement and is not optimized over the entire effective image area. That is, constant light intensity correction is unchanged at an area that does not fulfill the determination requirement;
(2) The scheme of correcting temporary light-intensity correction data corrects temporary light-intensity correction data according to a fluctuation of the Wbi with respect to an average value Wave of Wi, and is just the same as constant spot size correction.
Therefore, this proposal is merely the combination of the constant light intensity correction and the constant spot size correction such that the constant light intensity correction is performed at some locations and the constant spot size correction is performed at some locations. Although the control amounts are constant, they are limited as per the constant light intensity correction and the constant spot size correction.
As described above, the constant correction (constant light intensity correction or constant spot size correction) cannot make the light intensity or the spot size to be converged merely to the resolution level of the amount of the current to be supplied that is predetermined as correction data, and the control amount is allowed to approach as close to a predetermined target value as possible, so that the value after constant correction with respect to each light emitting element fluctuates around the predetermined target value.
Therefore, the light intensity or the spot size after constant correction is not regular and does not take a plurality of light emitting elements into consideration; and
(3) The determination requirement of correcting temporary light-intensity correction data is stepwise with respect to Wbi, such as no correction made on the temporary light-intensity correction data when Wbi<10, correction made on only 4% of the temporary light-intensity correction data when 10≦Wbi<20 and correction made on only 8% of the temporary light-intensity correction data when 20≦Wbi. A sudden change occurs near that light emitting element whose Wbi is 10 or 20. FIG. 25 depicts that image. The broken line in the figure indicates data before correction, and the solid line indicates data after correction. As indicated by arrows in the diagram, steps are produced near the light emitting elements whose Wbi is 10 or 20. There is a concern that such a sudden change would adversely affect images. Furthermore, the correction does not take a plurality of light emitting elements into consideration.
Japanese Patent Application Laid-Open Publication No. 2002-127492 proposes a light intensity adjusting method that takes a plurality of light emitting elements into account. Specifically, the result of comparison (e.g., the inclination of an approximate line or the moving average) of the property values of a plurality of light emitting elements at a predetermined threshold in the exposure intensity distribution is controlled to lie within a preset range over the entire effective image area. This prevents the property values of nearby light emitting elements from changing abruptly.